System for transcutaneous monitoring of intracranial pressure

ABSTRACT

A system for measuring and converting to an observer intelligible form an internal physiological parameter of a medical patient. The invention allows transcutaneous telemetry of the measured information intracranial pressure via a system which includes a patient implanted sensor module and a processing and display module which is external of the patient and optically coupled to the sensor module via an external coupling module. A sensor within the implanted module transduces the measured information and a near infrared (NIR) emitter transmits this telemetry information when interrogated by the complementary external coupling module. Alternately, a set of tuned inductor-crystal circuits versus inductor-crystal comprised in part of a cylindrical crystal oscillator whose resonant frequency is sensed by a dipper circuit arrangement is provided. Power for the sensor module is derived inductively through rectification of a transcutaneously-applied high-frequency alternating electromagnetic field which is generated by a power source within the external coupling module, in concept much like a conventional electrical transformer. A computer within the processing and display module calculates the parameter value from the telemetry signal and represents this data either in numerical, graphical, or analog format.

RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No.11/065,428 filed on Feb. 24, 2005.

FIELD OF INVENTION

The present inventions relate generally to transcutaneous telemetry withan implantable biomedical device, and more specifically relate to asystem for transcutaneous monitoring of intracranial pressure (ICP).

BACKGROUND OF THE INVENTION

The measurement of intracranial pressure (ICP) plays a critical role inseveral neurosurgical conditions. Various pathological processes such ashydrocephalus, tumors, and trauma can cause alterations in the pressurewithin the skull. If not adequately controlled, increases inintracranial pressure (due to accumulation of cerebrospinal fluid, bloodclots, tumors, or brain swelling) can cause secondary damage tootherwise healthy brain tissue.

A number of technologies currently exist to monitor brain pressure. Manyof these rely on invasive techniques with percutaneously implantedsensors. Wires or fiber optic cables are often used to transducepressure information from electromechanical or optomechanicaltransducers, which relegates these technologies to short term use. Atthe end of use these sensors are withdrawn from the body. Severaldisadvantages are associated with such devices: 1) the presence of apercutaneous probe increases the chance of iatrogenic infections such asmeningitis and cerebritis; 2) the probe must be withdrawn at the end ofuse and so it is not reusable for subsequent episodes of suspectedintracranial hypertension such as with hydrocephalus; and 3) thepercutaneous cable is subject to mechanical failure and to inadvertentpull-out during routine patient care.

In an attempt to mitigate these disadvantages, numerous investigatorshave tried to develop non-invasive techniques for monitoringintracranial pressure. Such methods have employed mathematicalcorrelations between physiological variables which can be transducedextracorporally such as blood pressure, heart rate, Doppler ultrasoundof cerebral blood vessels, near-infrared (NIR) spectroscopy of cerebraloxygenation, retinal imaging, etc. While some success has been achievedin monitoring trends in ICP, no method has been fully successful inderiving the absolute intracranial pressure, and these known techniqueshave not gained significant clinical utility for monitoring ICP.

In another aspect of the present invention, a passive device based ontwo quiescent resonant tuned circuits is positioned subcutaneously onthe patient's skull. While the concept of using a passive device forintracranial pressure monitoring exists in the prior art via Seylar,U.S. Pat. No. 4,114,606, a number of novel improvements are brought tobear in the present invention. In one such improvement, the devicecompensates for temperature, aging and stray capacitance by using twocollocated and implanted circuits, the first circuit in contact with theintracranial space and thus experiencing the intracranial pressure; thesecond circuit sealed at a fixed and predetermined pressure. In anothersuch improvement, the present invention incorporates an radio frequencyidentification (RF-ID) tag for holding calibration data and othercritical data such as patient information and insertion date.

The presently described invention utilizes near-infrared beams totraverse biological tissue for the digital transmission of data.

Physiological parameters such as tissue oxygenation may be measured bycomparing the absorption of specific optical wavelengths by thehemoglobin and cytochrome chromophores. This technology is omnipresentin the hospital setting in the form of pulse oximetry. In what is bestdisclosed as spectrophotometry, these aforementioned measurementtechniques utilize analog means to derive quantitative measures of somephysiological parameter via absorption of selected spectra. In adramatic paradigm shift, the presently described invention utilizes aninfrared beam to traverse biological tissue for the digital transmissionof data.

The suitability of transmission of data across biological tissues viainfrared beam is dependent primarily upon the attenuation of the lightbeam. From the modified Beer-Lambert equation, the attenuation,expressed in optical density, is:Attenuation(OD)=−log(I/Io)=Bμ _(a) d _(p) +G   (1)

Where “I” represents the transmitted light intensity, “Io” representsthe incident intensity, “B” is a path length factor dependent upon theabsorption coefficient “μ_(a)” and scattering coefficient “μ_(s)” “dp”represents the interoptode distance, and G represents ageometry-dependent factor.

The Near Infrared (NIR) spectrum is generally referred to as thefrequency range from 750 to 2500 nm. In vivo measurements of NIRabsorption during transillumination of the newborn infant brain suggestan optical density of 10 over interoptode distances of 8-9 cm. See Cope,M and Delpy, D. T. “System for long term measurement of cerebral bloodand tissue oxygenation on newborn infants by near infraredtransillumination.” Medicine, Biology, Engineering and Computing, 26(3):289-94, 1988. Assuming the light source and detector are collinearand antiparallel, the geometry-dependent factor, G, becomes negligible.Because biological tissue is an effective multiple scatterer of light,the effective path length traveled by a given photon can only beestimated. In a study measuring the water absorption peak at 975 nm andassuming average tissue water content, the path length of brain tissueis estimated at 4.3 times the interoptode distance. See Wray, S., Cope,M., Delpy, D. T., Wyatt, J. S. and Reynolds, E. O. R. “Characterizationof the near infrared absorption spectra of cytochrome aa3 and hemoglobinfor the non-invasive monitoring of cerebral oxygenation.” BiochimicaBiophysica Acta 933:184-92, 1988. Thus, from the Beer-Lambert equations,the calculated absorption coefficient for human brain is approximately0.26 cm² with an assumed path length of 4.3. This is within the range ofabsorption coefficients (0.0434-0.456 cm2) quoted in the literature. SeeSvaasand, L. O. and Ellingsen, R. “Optical properties of brain.”Photochemistry and Photobiology, 38 (3):293-9, 1983. In the studies ofTamura and Tamura, extracranial structures such as skin, muscle and bonehad minimal effects on the NIR transmission-mode absorbance, presumablybecause the blood flow and oxygen consumption of these structures is lowcompared to that of cerebral cortex. See Tamura, M. and Tamura, T.“Non-invasive monitoring of brain oxygen sufficiency on cardiopulmonarybypass patients by near-infra-red laser spectrophotometry.” Medical andBiological Engineering and Computing 32:S151-6. The relatively minorcontribution of scalp tissue to NIR absorption is further corroboratedby Owen-Reece, Owen-Reece, H., Elwell, C. E., Wyatt, J. S. and Delpy, D.T., “The effect of scalp ischaemia on measurement of cerebral bloodvolume by near-infrared spectroscopy.” Physiological Measurements,November, 17 (4):279-86, 1996. Thus, it is reasonable to expect that fora typical scalp thickness of 1 cm, the absorption would be somewhat lessthan: Bμ_(a)d_(p)=(4.3)(0.26)(1)=1.1, assuming that the geometry factoris negligible. Therefore, with an attenuation of one to two orders ofmagnitude and an NIR emitter output power of 5 mW, the transmitted lightintensity is well within the sensitivity range of common siliconphotodiodes.

Delpy, et al have investigated the relationship between attenuation andthe transit time of light through tissue in an attempt to determineoptical path length. See Delpy, D. T., Cope, M., van der Zee, P.,Arridge, S., Wray, S. and Wyatt, J. “Estimation of optical path lengththrough tissue from direct time of flight measurement.” Physics,Medicine and Biology 33 (12): 1433-42, 1988. Temporal dispersionresulting from spatial and temporal delta functions of the input beam asit passes through scattering tissue may be described by the temporalspread point function (TSPF). Using a Monte Carlo model of lighttransport in tissue and experimentally derived (in vitro rat brain)scattering phase function at 783 nm, they computed the TSPF for a beamof light passing through a 1 cm thick slab of brain tissue. Estimates ofpath length based upon the time-of-flight of photons using the TSPFintegrated over the exit surface, at all exits angles, and assumingradial symmetry, yields an average path length of 5.3 times theinteroptode distance. The final photons to emerge from the tissue arecalculated to have traveled 9.2 times the interoptode distance. Thetemporal dispersion of the light will limit the maximum transmissionbandwidth:F_(max)=1/t=c/dn   (2)

Where F_(max) is the maximum transmission frequency, t is the time forthe light to traverse the tissue, c is the speed of light, d is thedistance traveled, and n is the refractive index.

SUMMARY OF INVENTION

In one of the embodiments of the present invention, systems and methodsare used which allow ad lib transcutaneous telemetry of absoluteintracranial pressure via a system which includes a patient implantedsensor module and a processing and display module which is external ofthe patient and optically coupled to the sensor module via an externalcoupling module. The sensor module is implanted in much the same fashionas with existing technologies but the skin is closed back over thedevice and no cabling penetrates the skin. A sensor within the implantedmodule transduces absolute pressure information and a near infrared(NIR) emitter transmits this telemetry information when interrogated bythe complementary external coupling module. Light in the near-infraredspectrum is easily transmitted through the skin and is detected by theexternal module. Indefinite longevity and small size is attained in theimplant by not incorporating a power source within the module. Instead,power is derived inductively through rectification of atranscutaneously-applied high-frequency alternating electromagneticfield which is generated by a power source within the external couplingmodule, in concept much like a conventional electrical transformer. Acomputer within the processing and display module calculates theabsolute pressure from the NIR telemetry signal and represents thesedata either in numerical, graphical, or analog format.

The present inventions overcome disadvantages of existing technologiesby providing a means for telemetric conveyance of physiological data viatranscutaneous projection of a near infrared light beam. The use of thistechnique for telemetry of intracranial pressure is only one of manypotential applications and any reference to intracranial pressuremonitoring is not meant to limit the scope of applicability.Furthermore, the transcutaneous telemetry of information is not limitedto a unidirectional fashion. Indeed, telemetric data may be transferredbi-directionally between an extra corporeal device and an implanteddevice. Broadly stated, the implanted device may be a sensor of one ormore physiological parameters, contain some form of data unique to thedevice or unique to the person (or organism) harboring the implant, ormay somehow monitor the physiological state of the person (or organism).Specific examples of devices include, but are not limited to,intracranial pressure monitors, tissue oxygen sensors, glucose sensors,neurostimulators, pacemakers, and defibrillators. An extra corporealdevice allows recording, display, or interpretation of data from theimplanted device and may communicate in bidirectional fashion to conveyinformation back to the implant such as calibration data, handshakingdata, etc.

The preferred embodiment of the present invention discloses animplantable biometric sensor system comprising a rigid subcutaneouscase, an access portal to an internal fluid in the subcutaneous case, adual frequency crystal oscillator, affixed to the rigid subcutaneouscase and in ducted communication with the internal fluid, for sensing apressure difference in the internal fluid, and at least one driver coilin electrical communication with the dual frequency crystal oscillator.

The preferred embodiment of the present invention also discloses amethod of determining intracranial fluid pressure providing the steps ofproviding a cylindrical crystal oscillator probe having a transducersection and a reference section, providing at least one driver circuitwithin the cylindrical crystal oscillator probe connected to thetransducer section and the reference section, exposing the referencesection to a reference pressure, exposing the transducer section to theintracranial fluid pressure, exciting the transducer section to providea first frequency energy absorption, exciting the reference section toproduce a second frequency energy absorption, measuring the firstfrequency energy absorption, measuring the second frequency energyabsorption, comparing the measurement of the first frequency energyabsorption and the second frequency energy absorption to determine adifference, relating the difference to the reference pressure todetermine the intracranial fluid pressure, and reporting theintracranial fluid pressure.

BRIEF DESCRIPTION OF DRAWINGS

The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1 is a schematic block diagram of a transcutaneous monitoringsystem in accordance with the present invention.

FIG. 2 is simplified coronal cross sectional diagram illustrating howthe sensor module may be implanted in a typical use with a patient.

FIG. 3 a is a simplified side elevational view of an implanted sensormodule in accordance with the invention.

FIG. 3 b is top plan view of the device of FIG. 3 a.

FIG. 4 is a schematic longitudinal cross sectional view of the implantedsensor module.

FIG. 5 a is a plan view of the upper side of a crystal which may be usedin one embodiment of the present invention.

FIG. 5 b is a plan view of the lower side of the crystal shown in FIG. 5a.

FIG. 6 is a schematic electrical block diagram illustrating a preferredarrangement for measuring a patient pressure parameter in accordancewith the invention.

FIG. 7 is an electrical schematic diagram depicting further details ofthe arrangement shown in FIG. 6.

FIG. 8 is a schematic block diagram of the electronics used in thesystem of the invention which are external to the patient.

FIG. 9 is a schematic longitudinal cross-section illustrating a sensingdevice for an embodiment of the invention which is suitable formonitoring brain tissue oxygenation.

FIG. 10 a is a plan view of the upper side of a crystal which may beused in the implant module of the present invention.

FIG. 10 b is a plan view of the lower side of a crystal which may beused in the implant module of the present invention.

FIG. 11 is a cross-section of a preferred embodiment of the ICPtransducer implant.

FIG. 12 is a plan view of a preferred embodiment of the ICP implant.

FIG. 13 a is a cross section of a preferred embodiment of the ICPtransducer implant.

FIG. 13 b is a partial view of the sealed cylindrical enclosure of theinvention.

FIG. 14 is a schematic block diagram of a preferred embodiment of theinvention.

FIG. 15 a is a graph of dipper amplitude versus frequency.

FIG. 15 b is a graph of intracranial pressure versus frequency.

FIG. 16 is a schematic diagram of a dipper current of an embodiment ofthe present invention.

FIG. 17 is a cutaway view of the structure of a preferred embodiment ofthe invention.

FIG. 18 is a circuit diagram of an alternate embodiment of the implant14 where an inductive coil is shared between two crystal elements.

FIG. 19 is a graph of dipper amplitude versus frequency accounting forprotein deposits.

FIG. 20 is a graph of sensor frequency versus elapsed time accountingfor protein deposits.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiments (by way of example, and not of limitation).

One embodiment of the present inventions is illustrated by FIG. 1, whichshows a schematic block diagram of a transcutaneous monitoring system10. A physiological parameter 12, such as intracranial pressure, istransduced by an implant 14. The implant is buried beneath the skin 16of the patient. Information regarding the transduced physiologicalparameter 12 is converted to a digital form which modulates anear-infrared (NIR) emitter in the implant 14. The modulated NIRtelemetry signal 18 emanates from the implant 14, permeates the skin 16of the patient, and is detected by an external coupling module 20.

Power to the implant 14 is derived inductively through rectification ofa transcutaneously-delivered time-varying electromagnetic field 22 whichis applied by an external power source via external coupling module 20,in concept much like a conventional electrical transformer. Implant 14is powered only when in the vicinity of the external coupling module 20.A computer calculates the physiological parameter 12 from the NIRtelemetry signal 18 and represents these data either in numerical,graphical, or analog format.

A typical in vivo implementation is shown in the simplified longitudinalcross-sectional diagram of FIG. 2. A hollow ventricular catheter 26 isplaced surgically into a cerebrospinal fluid (CSF) filled ventricle 28of the brain 31. The CSF is communicated via the ventricular catheter 26to the implant 14. The physiological parameter 12, intracranialpressure, is sensed and transmitted via NIR telemetry signal 18 from theimplant 14, to the external coupling module 20, through the overlyingskin 16. In the scenario of a ventriculoperitoneal shunt, the CSF exitsthe implant 14 and passes, via a distal catheter 32, through a valveassembly (not shown) and ultimately to the peritoneal cavity of theabdomen (not shown). The implant 14 is installed superficial to, orembedded within, the skull 34. Dura 36 is depicted as an additionalanatomical landmark.

Choice of the preferred NIR wavelength for transcutaneous telemetrypursuant to the present invention is dependent upon the absorptioncoefficients of the intervening tissues. The absorption by melanosomesdominates over the visible and near-infrared spectra to about 1100 nm,above which free water begins to dominate. Absorption by the dermisdecreases monotonically over the 700-1000 nm range. Whole blood has aminimum absorption at about 700 nm but remains low over the 700-1000 nmrange. The nadir in the composite absorption spectrum therefore lies inthe 800-1000 nm range.

The actual wavelength utilized is therefore dictated by the optimalspectral range (as above) and the availability of suitable semiconductoremitters. Several suitable wavelengths may include, but are not limitedto: 760 nm, 765 nm, 780 nm, 785 nm, 790 nm, 800 nm, 805 nm, 808 nm, 810nm, 820 nm, 830 nm, 840 nm, 850 nm, 870 nm, 880 nm, 900 nm, 904 nm, 905nm, 915 nm, 920 nm, 940 nm, 950 nm, 970 nm, and 980 nm. Wavelengthsoutside this range may be used but will be subject to greaterattenuation by the intervening tissues.

Various designs for the implant 14 housing are suitable and arepresentative design is depicted in FIG. 3 a and FIG. 3 b whichrespectively depict simplified side elevational and top plan views ofthe device 38. Device 38 includes a cylindrical external housing 40having a fluid inlet port 42 and fluid outlet port 44 for cerebralspinal fluid (CSF). The external housing 40 must be biocompatible,rigid, and have a superficial face which is substantially transparent toNIR telemetry signal 18. A material such as polycarbonate (opticallytransparent to near infrared wavelengths) may be used.

The footprint of the implant in the preferred embodiment is round toaccommodate the implant's power supply coil. Furthermore, a roundfootprint allows the external housing 40 to be easily recessed into theskull 34 during implantation using a twist-drill. The seating flange 46limits the depth of the recess such that the seating flange 46 remainsflush with the skull 34 surface.

One preferred embodiment may include a reservoir access dome 48, or“Rickham reservoir,” which is an integrated self-sealing chamber made ofa material such as Silastic. A needle may be introduced percutaneouslythrough the skin 16 into the reservoir access dome 48 to allow access tocerebrospinal fluid within the implant 14, which in turn, communicateswith cerebrospinal fluid within the brain ventricle 28 via fluid inletport 42 and ventricular catheter 26. Alternate embodiments of theexternal housing 40 may include alternative locations for the fluidports, such as placement of the fluid inlet port 42 on the bottom ofexternal housing 40.

A schematic longitudinal cross sectional view of the implant 14 appearsin FIG. 4. Cerebrospinal fluid enters fluid chambers 52 via fluid inlet42. A transducer crystal 50 is mounted across external housing 40 to actas a diaphragm in contact with fluid chamber 52. In the preferredembodiment, the transducer crystal 50 is a quartz (silicon dioxide)crystal which exhibits a change in oscillation frequency as apredictably stable function of deformation, and hence pressure, ontransducer crystal 50. In FIG. 4 the transducer crystal 50 is depictedin the deformed state. The transducer crystal 50 may be composed of abiocompatible material (e.g. silicon dioxide) which will notsignificantly degrade in mechanical properties over the lifetime of theimplant. Important characteristics of the sensor to be employed aresensitivity, electromechanical stability, absolute pressure accuracy,biocompatibility and electrical noise immunity. The quartz crystaltransducer crystal 50 may be trimmed at the factory during fabricationto achieve calibration.

A reference crystal 54 is also housed within the implant 14. Thisreference crystal 54 is of identical construction to that of transducercrystal 50 and the difference in oscillation frequency between thesecrystals correlates directly with the deformation, and hence, pressureapplied to the transducer crystal 50.

A monolithic circuit 58 within the implant 14 contains the necessaryelectronics to operate the implanted sensor module. These electronicsact to modulate the output of a near-infrared emitter as a function ofpressure on the transducer crystal 50.

A puncture shield 56 serves to protect transducer crystal 50 from damagedue to needles introduced through the reservoir access dome 48. Thepuncture shield 56 as well as the transducer crystal 50 aresubstantially transparent to the NIR telemetry signal 18.

Transcutaneous telemetry from the implant 14 is transmitted optically tothe external coupling module 20 via NIR telemetry signal 18. In vivo,soft tissues are relatively permeable to wavelengths within the nearinfrared (NIR) spectrum. This permeability, coupled with specifichemoglobin absorption peaks, is exploited in non-invasive transcutaneousoxygenation monitors and NIR spectroscopy. In these applications it isthe relative absorption at specific wavelengths that is capitalizedupon, rather than the transmission of data over a tissue-permeablewavelength as in the present invention.

Analog signal transmission is not suitable due to the unpredictabilityof the NIR absorption by the skin 16. However, any one of numerousmethods for digital signal transmission may be incorporated. Existingserial data transmission protocols, whether synchronous or asynchronous,require complex electronics to encode the data. More simply, frequencymodulation or pulse-width modulation may be employed, particularly sincethe bandwidth of the physiological data is low. In the preferredembodiment, frequency modulation is used.

A computer within the processing and display module 24 calculates thephysiological parameter 12 from the NIR telemetry signal 18, as detectedby external coupling module 20, and represents these data either innumerical, graphical, or analog format.

One preferred embodiment of the invention employs a pressure transducercrystal 50 composed of an x-cut quartz crystal. In typical transducerapplications, mechanical deformations of a crystal are detected aspiezoelectric charges developed across the face of the crystal. Whilethis works well for time-varying signals, leakage currents render thistechnique inapplicable to measurement of static or slowly-changingdeformations of a crystal.

An alternate approach is to resonate an x-cut crystal at its fundamentalfrequency; mechanical deformation of the crystal, such as due to anapplied pressure, will alter the resonant frequency in a predictablefashion. The pressure applied to the crystal face is thus calculated bymeasuring the change in crystal oscillation frequency. This technique isapplicable to both static and dynamic measurements and is extremelystable as a function of time.

The sensitivity of a crystal acting as a pressure-sensitive diaphragm isdependent upon its stiffness and mounting configuration. To achievemaximal sensitivity, the crystal should be as thin as possible, yetadequately robust to withstand the pressure requirements of theapplication without exceeding the crystal's burst pressure.

The pressure transducer crystal 50 is in contact with cerebrospinalfluid within the fluid chamber 52. The chemical composition of quartz(silicon dioxide) has been demonstrated to be biocompatible and haveminimal biofouling. Biofouling is further minimized by surface polishingof the crystal surface during manufacturing. Long-term resonantfrequency stability is theoretically ensured despite biofouling due tothe flexural stiffness of the crystal being orders of magnitude greaterthan that of surface contaminant proteins.

In one preferred embodiment, a gold (Au) coaxial electrode pattern isdeposited onto the crystal as shown in FIGS. 5 a and 5 b, which arerespectively top 60 and bottom 62 plan views of the crystal. The topside is the biofluid side; the bottom-side 62 is where electricalcontact is made. Gold has also been demonstrated to be biocompatible andhave minimal biofouling. By lapping gold around the edge from the top 60to the undersurface at bottom 62 of the crystal, the surface at 60 incontact with the cerebrospinal fluid can be made entirely referenced atground potential. No electrical connections 70 are in contact with theCSF as electrodes from each face of the crystal are available on theundersurface 62 of the crystal and are separated by an inter-electrodegap 72. Slots 64 may be etched in the gold electrode surface to reduceor eliminate eddy currents from forming, hence improving power couplingfrom the external coupling module 20 to the implant 14. Additionally, anIR transmission port 68 may be left without metallization to allowtransmission of infrared light through the crystal.

Pressure on the transducer crystal 50 will cause the oscillationfrequency to decrease. Consequently, to ensure a monotonic increase indifferential frequency with increasing pressure, it is necessary fornominally identical transducer crystal 50 and reference crystal 54 to bematched such that the transducer crystal 50 has the lower naturalfrequency of the pair. Alternatively, the transducer crystal 50 may bedesigned to be nominally lower in frequency than the reference crystal54 to increase the temporal resolution of the system, but at the expenseof immunity to frequency drift.

In the preferred embodiment, the sensitivity and long-term stability ofthe system is maximized using a frequency-coherent detection scheme. Asshown in FIG. 6, two identical, yet independent, crystal oscillators areemployed. A reference crystal 54 serves a reference oscillator 74 whilethe transducer crystal 50 serves the transducer oscillator 76. Thedifference in frequency between the two oscillators is detected using aheterodyne amplifier 78. The difference frequency is ultimately measuredand used to compute the pressure applied to the transducer crystal 50.The inherent long-term stability of the quartz crystal-controlledtransducer oscillator 76 is augmented by cancellation of drift (thermal,aging, parasitic capacitance, etc.) by the reference oscillator 74.

The output of the heterodyne amplifier 78 is low-pass filtered to obtainthe beat-frequency and a level detector 80 with hysteresis is used toderive a digital signal to trigger a monostable multivibrator 82 at thebeat frequency. The output of the monostable multivibrator 82 is used tomodulate the NIR-emitter diode 86 via driver 84. The output pulse of themonostable multivibrator 82 is selected to be as short as feasible tominimize the power consumption of the implant. The system is designedsuch that lower, more physiological pressures, are associated with lowerbeat frequencies, again to decrease current consumption. As intracranialpressure rises, the beat frequency increases. The dynamic range of thefrequency change is determined by the electromechanical characteristicsof the transducer crystal 50 over the operating pressure range. Atwo-point calibration of the implant 14 may be performed at the factoryby trimming of the components on the monolithic circuit 58. The minimumon-time pulse width for the NIR-emitter diode 86 is typically limited bythe bandwidth of the detector electronics in the external couplingmodule 20.

Various semiconductor materials are known which are capable of emittingsuitable NIR wavelengths. In practice, most are light-emitting diodes(LEDs). The light output intensity is generally proportional to thediode's forward current, and depending on the device, this current cantypically range from 20 mA to 1.5 A. Laser diodes tend to have greateroptical output but at the expense of higher current requirements andmore complicated driver circuitry. High current requirements are notfeasible in a miniature implanted device which relies ontranscutaneously derived power.

The Vertical Cavity Surface Emitting Laser (VCSEL) provides ahigh-performance, low-current, high-optical-power solution. In thepreferred embodiment, a VCSEL is employed as the NIR-emitter diode 86,such as a Honeywell SV5637 VCSEL laser diode which produces an 850 nm1.25 mW/cm output at a mere 10 mA forward current. Furthermore, with thevertical cavity design, the light beam radiates perpendicular to thewafer surface. This facilitates the fabrication of the laser diode andthe remainder of the implant 14 electronics on a microminiaturizedmonolithic circuit 58.

FIG. 7 depicts a preferred embodiment of the implant 14 circuitry.Referring also to FIG. 6, transistors Q1 and Q2 compose two identicalColpitts crystal oscillators, 76 and 74, respectively. XI is thetransducer crystal 50 of FIG. 6 while X2 is the reference crystal 54. XIand X2 may have the same nominal resonant frequency or may bedeliberately tuned with a small offset. Due to these oscillators beingessentially identical, the long-term drift, thermal drift, and voltagedependence cancels.

The outputs of each oscillator 76 and 74 are ac-coupled via capacitorsC5 and C6 to a heterodyne amplifier 78 (FIG. 6) composed of Q3. Thelow-pass filtered (R13 and C7) heterodyne signal has a fundamentalfrequency equal to the frequency difference between the two oscillators76 and 74. The heterodyne signal is dc-coupled to Q4 and Q5 which areconfigured as a programmable unijunction-transistor voltage comparatorand this serves as a level detector 80. The set-point of the comparatoris determined by the voltage divider composed of R14 and R15.

The output of the unijunction transistor pair provides a digital signalwhich turns the NIR emitter diode 86, laser diode D4, on and off at thedifference frequency of the two oscillators. A monostable multivibrator82 (FIG. 6) is composed of Q6, Q8, C8 and associated resistors. When theinput voltage is below the unijunction set-point, transistor Q6conducts, allowing capacitor C8 to charge through resistor R18. Thevalue of C8 is selected to provide adequate charge to drive laser diodeD4 at the desired forward current for a nominal minimum period. R18 isselected to provide adequate charging current during one cycle whileminimizing current drain on the power supply. Peak laser diode forwardcurrent is regulated by Q7. When the voltage-comparator input exceedsthe threshold voltage, Q6 turns off to isolate the current drain of thelaser diode from the supply rail, while Q8 conducts current from C8 tothe laser diode D4. The duration that the laser remains on is determinedby the values of C8, R19, and the minimum forward lasing current of D4.Current consumption is minimized by keeping the duty cycle of the laserdiode low.

Power to the implant 14 is inductively coupled to coil LI via atime-varying electromagnetic field 22 (FIG. 1) which is appliedtranscutaneously by the complementary external coupling module 20. CoilLI may be a wire wound as a ‘short solenoid’ which is embedded in theimplant's external housing 40 (FIG. 3 a), or as in a preferredembodiment, a photochemically-etched metallic spiral on a suitablesubstrate such as the monolithic circuit 58 (FIG. 4). The inducedelectromotive force from center-tapped coil LI is then rectified bydiodes Dl and D2, which are ideally of the Schotkey type. This producesa direct current (DC) which is subsequently low-pass filtered byresistor R22 and capacitors C9 and C10 to derive a DC supply voltage. Azener diode D3 across the output is used to suppress voltage transientswhich might be induced by extraneous magnetic fields, such as from aMagnetic Resonance Imaging (MRI) scanner.

Most of the external electronics may be conveniently located in ahousing mounted at the bedside of the patient. FIG. 8 depicts aschematic block diagram of the electronics external to the patient. Anexternal coupling module 20 houses electronic components which arenecessarily closely associated with the implant 14. A single cable (notshown) goes from the external coupling module 20 to the processing anddisplay module 24. This cable is shielded to minimize spuriouselectromagnetic radiation emission. The external coupling module 20 isplaced in proximity to, i.e. over, the implant 14 to telemeter thephysiological parameter 12. The external coupling module 20 may bedisc-shaped and contains a coil 88 to deliver inductively-coupled powerto the implant 14. An optical bandpass filter 90 on the undersurface ofthe external coupling module 20 permits NIR telemetry signal 18 (FIG. 1)to reach a semiconductor photodetector 92. In a preferred embodiment, aphotodiode is used. Ideally, this device is matched with the NIR emitterdiode 86 such that the peak wavelength sensitivity of photodetector 92corresponds to that of the NIR emitter diode 86. Optical signal-to-noiseratio (SNR) is improved using a narrow optical bandpass filter 90.Further improvements in SNR may be achieved through biasing of thephotodiode. Advanced techniques such as phase-coherent orfrequency-coherent detection may be employed.

The external coupling module 20 may optionally contain a preamplifier 94for the photodetector 92. The photodetector 92 signal is furtherconditioned by an automatic gain control (AGC) amplifier 96. In thepreferred embodiment, an edge-detector such as a Schmitt trigger 98 isused to detect the rise and fall of the photodetector 92 output, whichin turn correlates with NIR emitter 86 pulse frequency. A microprocessor100 converts the pulse frequency to a pressure value based on knowncalibration constants. The microprocessor 100 may then perform anyadditional signal processing prior to outputting the pressure dataeither graphically, numerically on a visual display 102, or in analogfashion via digital-to-analog converter 104. A calibrated analog outputfacilitates connection to existing patient-care monitoring equipment.

A high-frequency oscillator 106 and associated power amplifier 108provide the necessary drive current to coil 88 to produce thetime-varying magnetic flux 22 (FIG. 1) to power the implant 14.Isolation transformer 110 provides galvanic isolation between theprocessing and display module 24 and the patient-connected externalcoupling module 20.

A visible bi-colored LED 111 mounted on the external coupling module 20(FIG. 1) casing aids in the positioning of the external coupling moduleover the implant 14. The LED indicates red when power is applied toexternal coupling module 20 and indicates green when NIR telemetrysignal 18 (FIG. 1) is detected from the implant 14. Thus, the green LEDmay be used to aid in positioning of the external coupling module 20over the implanted sensor module as the NIR emitter diode 86 (FIG. 6)will only be detected when there is adequate proximity and collinearalignment of the coil 88 and secondary (LI of FIG. 7) coils. The outercasing of external coupling module 20 is optically opaque at the NIRemitter diode 86 wavelength to avoid exposure of medical personnel tothe optical radiation. The external coupling module 20 may be held inplace by any convenient means, such as with a headband, a stocking cap,or preferably by an articulated arm attached to the bedside.

Each pressure transducer system, e.g. transducer 50 and reference 54crystal pair, (FIG. 4) is factory calibrated using a two-pointcalibration. The difference frequency at zero gauge pressure is used asa baseline and the difference frequency at a specified physiologicalextreme, e.g. 100 mmHg, is used to compute the slope of the two-pointcalibration. These data are then sufficient to compute the actualtransducer pressure with high linearity and monotonicity given that thetransducer crystal 50 frequency is intrinsically lower than thereference crystal 54 frequency.

The calibration coefficients obtained in the above fashion may be storedelectronically in a database accessible from the internet. Uponinitialization of the pressure recordings from the transducer, thedatabase may be accessed and the proper calibration coefficients enteredinto the processing and display module 24. The database may be indexedby patient identifier.

The VCSEL laser diodes considered for use as the NIR emitter provide apower output of 5 mW or less. The amount of energy absorbed by theoverlying tissue would be well below the safety standard of 90 mW.Furthermore, the design described inherently has a ‘safety interlock’ aspower is only applied to the implanted sensor module when the externalcoupling module is held directly over the implant 14. The NIR-opaqueexternal coupling module 20 prevents an observer from gazing into thelaser beam emanating from the implant. A visible light photo detectormay be embedded in the patient-side of the external coupling module 20to prevent the device from being energized when ambient light ispresent. This necessitates that the external coupling module be appliedto the scalp overlying the implant 14 (FIG. 1) prior to telemetrycommencing.

The preferred embodiment described in the foregoing provides a simpleand practical means for unidirectional transcutaneous telemetry of aphysiological parameter using near infrared light. However, thedirection of information travel is immaterial. If both theintracorporeal and extracorporeal devices are equipped with atransceiver (i.e. emitter and detector), then data may flow inbidirectional fashion.

Two different wavelengths of infrared light may be utilized to minimize“crosstalk” during communication between the intra- and extra-corporealdevices. The choice of transmission wavelength is dependent upon thepermeability of the tissue at that wavelength and the electricalcharacteristics of the semiconductor emitter. Appropriate choices couldinclude, but are not limited to, 850 nm and 1050 nm. Because biologicaltissues tend to scatter incident light in unpredictable fashion, it isconceivable that light from an emitter in either device could bereflected back upon the receiver in that same device. By specifying aparticular wavelength for transmission in a given direction, thereceivers may be equipped with narrow band-pass filters to selectivelyrespond only to incident light from the intended sender. Furthermore,incorporation of such band-pass filters gives the desired effect ofexcluding ambient light which could adversely affect the signal-to-noiseratio of the communication pathway.

Existing electronic implant devices typically utilize radio frequency(RF) telemetry during application of a strong magnetic field (to actuatea reed switch); narrow bandwidth optical telemetry would markedly reduceor eliminate the susceptibility of these devices to the electromagneticfields experienced during Magnetic Resonance Imaging.

In an alternative embodiment, the functionality of the implant ismaximized by incorporating a microprocessor into the implanted device.While increasing device complexity, it allows for complex datatransmission schemes, signal processing within the device, storage andmodification of calibration data, and a broader information transmissionbandwidth.

Also, the NIR emitter may serve a dual role. Physiological pressures,such as that of cerebrospinal fluid, may be measured using the sameinfrared emitter as used for the transcutaneous telemetry of data. Anoptical means of pressure measurement could involve the use of areflective strip on a distensible membrane which is in contact with thecerebrospinal fluid. The displacement of the membrane is considered afunction of pressure. Hence, by measuring the degree of displacement ata given location on the membrane, the applied pressure may becalculated. A portion of the light emitted by the incorporated NIRemitter is bounced off the reflective area of the membrane and theresultant reflection pattern is detected by a linear array of photodetectors such as a charge-coupled device. Alternatively, a diffractiongrating may be utilized and the resultant interference pattern analyzed.

The inclusion of a semiconductor temperature sensor within the implantelectronics would allow temperature compensation for variations inambient temperature. This is particularly important with opticalpressure transduction schemes utilizing diffraction pattern analysis dueto the high sensitivity of such systems to dimensional changes fromthermal expansion.

The transcutaneous telemetry of data via infrared light beam serves asthe basis for a plethora of applications. This technology may serve as areplacement for existing radio frequency (RF) telemetry systems(incorporated in cardiac pacemakers and neurostimulators) which may beaffected by environmental RF energy such as present in MRI scanners.

Furthermore, complex serial data transmission protocols are facilitatedby the high bandwidth, allowing many physiological parameters to betransduced simultaneously in real-time.

A logical extension of the technology described herein is incorporationof a photometric system for measuring brain tissue oxygenation. Thetechniques for spectrophotometric measurement of total hemoglobin,oxyhemoglobin, and deoxyhemoglobin are described in the literature. Insummary, tissue absorption at several near infrared wavelengths (e.g.780 nm, 805 nm, 830 nm) is used to compute the concentration of eachchromophore.

An illustrative system for implementing ICP and brain tissue oxygenationmonitoring is depicted in FIG. 9. Portions of the implant which areidentical to that in FIG. 4 are identified by the same referencenumeral. In this system, a microprocessor is used within the implanteddevice to perform data analysis and facilitate bidirectionaltranscutaneous NIR communication. Separate wavelengths are used for eachof the communication send and receive channels.

In FIG. 9, a brain oxygenation sensor is comprised of NIR emitters 112of the appropriate wavelengths (e.g. 780 nm, 805 nm, 830 nm) which areoriented such that their light beams are directed downward throughNIR-transparent windows in the base of the implant housing. Powerconsumption by the NIR emitters 112 is minimized through multiplexing;briefly turning on each emitter sequentially at a rate fast enough tomake the physiological parameter 12 of interest relatively quasistatic.A fiber optic catheter 118 extends from the implant housing into thebrain tissue and conveys transmitted NIR light 120 from the multiplexedNIR-emitters 112 back to an optical detector 114 to measure opticalabsorbance. An optically opaque sheath 116 covers all but a smallportion of the tip of the fiber optic catheter 118 such that the lightmust travel a minimum known distance through the brain tissue.Simultaneous linear equations available in the literature relate therelative absorbance of light at each wavelength tospectrophotometrically calculate the concentration of total hemoglobin,oxyhemoglobin and deoxyhemoglobin. These calculations may be performedby a microprocessor embedded into the implanted device. Themicroprocessor device also manages asynchronous serial datacommunications with the extracorporeal monitor via a bidirectionaldual-wavelength NIR telemetry signal 18 providing handshaking, sendingof the chromophore and ICP readings, and receiving of calibrationconstants.

In another preferred embodiment, a gold (Au) coaxial electrode patternis deposited onto the crystal as shown in FIG. 10 a and FIG. 10 b, whichare respectively top 202 and bottom 204 plan views of the crystal. Topside 202 is the biofluid side; bottom-side 204 is where electricalcontact is made. Gold has also been demonstrated to be biocompatible andhave minimal biofouling. By lapping gold around the edge from the top202 to the undersurface at bottom 204 of the crystal (shown as 214), thesurface at 202 in contact with the cerebrospinal fluid can be madeentirely referenced at ground potential. No electrical connections 210are in contact with the CSF as electrodes from each face of the crystalare available on the undersurface 204 of the crystal and are separatedby an inter-electrode gap 206. A single slot 208 is etched in the goldelectrode surface of bottom 204 and a slot 209 is etched in top 202 inorder to reduce or eliminate eddy currents from forming and henceimproving power coupling from the external coupling module 20 to theimplanted sensor module 14. Additionally, an IR transmission port 200may be left without metallization to allow transmission of infraredlight through the crystal.

A cross-section of a preferred embodiment of the ICP transducer implantis shown in FIG. 11. A generally cylindrical stainless steel housing 302is provided which is gold flashed. Housing 302 includes threaded opening331 and threaded opening 309. Housing 302 further includes seating ring318, seating ring 316 and seating ring 314. Each seating ring,respectively, is machined into the inside of the housing and iscomprised of a generally annular ledge for support of the internalcomponents of the implant. Transducer crystal 304 is seated adjacentseating ring 318 and held in place by circumferential fillet 354.Circumferential fillet 354 is a gold fillet and attaches to both thetransducer crystal and seating ring 318 to firmly form a mechanical bondbetween transducer crystal 304 and housing 302. Integrated monolithiccircuit 308 fits within seating ring 316 and is held in place bycircumferential fillet 352. Circumferential fillet 352 is gold and formsa mechanical bond between integrated monolithic circuit 308 and housing302.

Reference crystal 306 is seated within seating ring 314 and held inplace by circumferential fillet 350. Circumferential fillet 350 is alsogold.

Housing closure 399 is a gold flash stainless steel disk which includesannular threads. Housing closure 399 is threaded into threaded opening309 in housing 302 and forms a hermetical seal to the interior ofhousing 302.

FIGS. 11 and 12 show an alternative embodiment of the ICP transducerimplant 1100. Housing cap 330 is a generally cylindrical structurecomprised of seating ring 339, threaded exterior 340 fluid inlet 332,fluid outlet 333 and optical opening 342. A silastic dome 335 is fixedon the exterior surface 336 of housing cap 330. Silastic dome 335 isfixed to exterior surface 336 with a silastic RTV compound. Fluid inlet332 and fluid outlet 333 are generally rectangular ports machined in aradial fashion in exterior surface 336. Fluid inlet 332 and fluid outlet333 are in ducted communication with interior chamber 337. Silastic dome335 is a NIR transparent flexible material capable of penetration byneedles for extraction of fluid from internal chamber 337 when thetransducer implant is in use. Shield 341 is fitted within seating ring339 and held in place by circumferential fillet 338. Shield 341 is an IRtransparent material capable of withstanding needle sticks withoutpenetration. Threaded exterior 340 is threaded into threaded opening331, affixing housing cap 330 adjacent transducer crystal 304. Shield341 includes holes 349 around its perimeter which provide ductedcommunication between interior chamber 337 and the top surface oftransducer crystal 304.

Transducer crystal 304 is held in electrical connection with integratedmonolithic circuit 308 through connector 312. Reference crystal 306 isheld in electrical connection with integrated monolithic circuit 308through connector 310. Integrated monolithic circuit 308 is providedwith infrared LED 320. Infrared LED 320 is positioned directly beneathtransducer crystal 304 in a position to emit radiation throughtransducer crystal 304, shield 341, and silastic dome 335, toward theexterior of the implant for reception and decoding of infrared signals.

Referring to FIG. 13 a, an alternate embodiment of the ICP transducerimplant is shown at 1300. In this embodiment, a stainless steel housing302 is provided. The stainless steel housing is gold flashed forbioinertness. In this embodiment, seating ring 376 is provided on theinterior of housing 302. Adjacent seating ring 376 is sealed cylindricalenclosure 370. Transducer crystal 305 is seated adjacent a seating ringand held in place by a circumferential fillet.

Referring then to FIG. 13 b, sealed cylindrical enclosure 370 includesseating ring 394 and threaded opening 391. Upper fixed pressure chamber393 and lower fixed pressure chamber 395 are provided connected bylongitudinal channel ducts 392. Reference crystal 371 is placed withinupper fixed pressure chamber 393 and adjacent seating ring 394.Reference crystal 371 is held in place by annular fillet 397. Channelducts 392 surround the circumference of reference crystal 371 andprovided ducted communication between upper fixed pressure chamber 393and lower fixed pressure chamber 395. An electrical connector 396 isprovided on reference crystal 371. Enclosure cap 390 is a cylindricaldisk having annular threads. Enclosure cap 390 is threaded into threadedopening 391 in sealed cylindrical enclosure 370. External electricalconnection is provided to reference crystal 371 via electrical connector396 and external connector 398 in enclosure cap 390.

Referring to FIG. 13 a, sealed cylindrical enclosure 370 is seatedadjacent seating ring 376 and held in place by circumferential fillet377. In the preferred embodiment, housing closure 399 is in directmechanical contact with sealed cylindrical enclosure 370 pressing itfirmly against seating ring 376. External connector 398 provideselectrical connection with connector 311 in direct electrical contactwith integrated monolithic circuit 308.

In this preferred embodiment, the sealed cylindrical enclosure isprovided to isolate reference crystal 371 from the interior of housing302 to avoid any potential deflection from the fluid pressure ininternal chamber 337. The pressure in internal chamber 337 enclosure isatmospheric. In other embodiments the internal chamber is evacuatedduring manufacture providing a pressure of as close to zero psi aspossible.

In yet another preferred embodiment of transcutaneous monitoring of ICP,a quiescent sensor is employed in combination with a radio frequencyidentification (RF-ID) tagging device in the subcutaneous implant. Sucha subcutaneous implant 14 and corresponding external coupling module 20is shown in the block diagram of FIG. 14, where the implant 14 is placedunder skin 16, the external coupling module 20 is brought over skin 16and in the vicinity of implant 14, and the external coupling module 20percutaneously reads the parameter of interest, intracranial CSFpressure.

The implant 14 comprises two high Q tuned resonant circuits, sensorcircuit 520 for sensing pressure and reference circuit 530, and an RF-IDtagging device 542 for storing information. The resonant frequency ofsensor circuit 520 is ƒ₁ and the resonant frequency of reference circuit530 is ƒ₀. When excited by external time-varying electromagnetic fields,resonantly tuned sensor circuit 520 and resonantly tuned referencecircuit 530 will tend to oscillate at their respective resonantfrequencies, ƒ₁ and ƒ₀, with a very narrow function of frequency,typical of high Q frequency resonances. Tuned sensor circuit 520 iscomprised of sensor crystal 521 connected in series with inductive coil525. Sensor crystal 521 is in physical contact with a biologicalenvironment (intracranial CSF) and experiences pressure and temperatureequilibrium with that environment. Tuned reference circuit 530 iscomprised of reference crystal 531 connected in series with inductivecoil 535. Reference crystal 531 is pressure sealed from given biologicalenvironment and held at a fixed pressure Po (e.g. normal atmosphericpressure) while maintaining temperature equilibrium with givenbiological environment.

Inductive coil 525 and inductive coil 535 have 10 turns and 5 mmdiameter and are constructed with 27 AWG bondable polymer insulatedcopper wire.

In an alternate embodiment of implant 14 shown in FIG. 18, a singleinductive coil 536 is connected to magnetically controlled magneticswitch 537. Magnet switch 537 is connected to both sensor crystal 521and reference crystal 531 so that when magnetic switch 537 is set,sensor crystal 521 is connected in series with inductive coil 536 andwhen magnetic switch 537 is reset, reference crystal 531 is connected inseries with inductive coil 536.

The resonance frequencies ƒ₁ of sensor circuit 520 and ƒ₀ of referencecircuit 530 are dependent upon the pressure experienced by sensorcrystal 521 and reference crystal 531, respectively. The presentinvention functions to measure the resonant frequency difference, f₀-f₁,between the sensor circuit 520 and reference circuit 530 and throughknown relationships of resonance frequency difference and pressurechange, calculate the absolute pressure within the given biologicalenvironment.

Sensor crystal 521 and reference crystal 531 consist of a syntheticpiezoelectric crystal, such as lead-zirconate-titanate (PZT), shaped ina tubular configuration oriented along the crystalline x-axis. Withappropriate electrical excitation, such a crystal will tend to oscillatein a “hoop” mode, wherein the radius of the tube expands and contractsover each cycle. The resonant oscillation frequency of the crystal isdependent upon the tube wall thickness and is highly stable as afunction of time.

Piezoelectric crystals of this type have been successfully deployed inpressure measurement applications. The crystal's oscillation frequencydecreases linearly due to loading when pressure is applied by a fluid toeither the internal or external surface of the crystal. The crystalresonant oscillation frequency is a reproducible linear function ofapplied pressure. Fluidic coupling between suitably fabricatedpiezoelectric crystals and the brain parenchyma, or alternatively CSF,can allow accurate and reproducible transduction of intracranialpressure in a continuous or episodal way.

Incorporation of two tuned circuits into the implant 14 facilitateslong-term measurement accuracy; in particular, resonantly tunedreference circuit 530 experiences the same long-term environmentalchanges as tuned sensor circuit 520. Tuned reference circuit 530essentially compensates for resonant frequency changes associated withaging, temperature and stray capacitance.

In the preferred embodiment of the present invention, the sensor circuitand reference circuit are constructed with a cylindrical piezoelectricceramic of about 20 mm total axial length, where 10 mm of length is usedfor the sensor section and the remaining 10 mm of length is used forreference section. The diameter of the cylinder is about 2 mm. Thepiezoelectric material is type P-6C and can be obtained from muRataCorporation of Nagaokakyo-shi, Kyoto, Japan. An alternate crystalsupplier is Boston Piezo Optics, Inc. of Boston, Mass. The naturalresonance of such a crystal is approximately 200 kHz. Sensor crystal 521is put in series with inductor 525 of value 0.2 uH to create tunedresonant sensor circuit 520 with peak frequency of approximately 8 MHzand Q of about 2500. In another preferred embodiment, the crystal canhave a diameter of about 6 mm with a wall thickness of 0.5 mm

In the preferred embodiment, the operative parameters of inductive coils525 and 535 are: implant coil diameter: 5 mm; implant coil width andthickness: 2 mm; implant coil turns: 10; implant coil inductance: 2e-7Henries; and implant coil resistance: 0.004 ohm.

In the preferred embodiment, RF-ID tagging device 542 with non-volatilememory is incorporated into implant 14 to store calibration data as wellas other relevant pre-stored data such as serial number, implant dateand patient name. Microchip part number, MCRF452 is a suitable part forRF-ID tagging device 542, requiring connection to a single externalinductive coil but no additional external capacitor. Said externalinductive coil is also contained inside implant 14 but not shown in FIG.14. Further useful details on deploying MCRF452 and similar RF-IDdevices may be found in the Microchip MicroID® 13.57 Mhz System DesignGuide found at www.microchip.com.

Referring again to FIG. 14, the external coupling module 20 is comprisedof “Dipper” circuit 430 connected to inductive coil 425 which operatetogether to sense the resonant frequencies of tuned sensor circuit 520and tuned reference circuit 530, an analog to digital converter (ADC)435 to measure dipper 430 output voltage, a voltage controlledoscillator (VCO) 460 in combination with a digital-to-analog converter(DAC) 455 to provide an excitation signal for “Dipper” circuit 430, afrequency counter 450 to measure the driving frequency from VCO 460, areceiver (RCVR) 445 for interrogating RF-ID devices, a microprocessor440 for computation of pressure and for overall command and control ofexternal coupling module 20, and a readout device 480 for displayingresults.

The principle component of the external coupling module 20 is the“Dipper” circuit 430 which is known in the art as a “grid-dip” meter or“gate-dip” meter and well-known in the art of antenna and RF tunercalibration. “Dipper” circuit 430 functions to measure the RF energyabsorption of a nearby tuned circuit. In the present invention, both thesensor circuit 520 and reference circuit 530 of implant 14 form thenearby tuned circuit. RF energy from “Dipper” circuit 430 is coupled tothe tuned circuits of implant 14 via inductive coil 425.

Various schemes may be employed to scan the operating frequency of“Dipper” circuit 430. In the preferred embodiment of the presentinvention, microprocessor 440 digitally communicates a prescribedvoltage to DAC 455 which generates output signal 458. VCO 460 acceptssignal 458 and generates an oscillatory signal 465 at a known frequency(“Dipper” frequency) commensurate with signal 458 and outputsoscillatory signal 465 to drive “Dipper” circuit 430. A closed-loopfrequency feedback is provided by frequency counter 450, so thatmicroprocessor 440 reads the “Dipper” frequency from frequency counter450 and adjusts DAC 455 to match the desired “Dipper” frequency.Microprocessor 440 may also log said frequency. The “Dipper” frequencyis swept across the expected operating frequencies of the implant 14.Utilizing ADC 435, the analog “Dipper” amplitude 433 output of “Dipper”circuit 430 is converted to digital form 434. Microprocessor 440 acceptsdigital form 434 of “Dipper” amplitude 433 from ADC 435 and processesthe data to effectively measure “Dipper” amplitude 433 as a function of“Dipper” frequency.

In an alternate embodiment, which operates open loop, DAC 455 is made tooutput a voltage ramp and microprocessor 440 in conjunction withfrequency counter 450 logs the resulting “Dipper” frequency as afunction of time. In a similar open-loop embodiment, DAC 455 is replacedby a suitable sawtooth voltage oscillator continuously operating at afrequency of about 10 Hz.

RCVR 445 reads pre-stored calibration data from RF-ID tagging device 542and sends it to microprocessor 440 which uses said calibration data fromthe RF-ID tag along with the measured “Dipper” frequency and measured“Dipper” amplitude 433 to compute an ambient pressure sensed by implant14 and exerted on sensor crystal 521. Microprocessor 440 formats theresults appropriate for display and sends the data to readout device 480via a flexible electrical cable 475. In one preferred embodiment, thereadout device may be physically integrated with the external couplingmodule 20, as for example, a liquid crystal display (LCD) screenattached to it. In a second preferred embodiment, the readout device 480is separated physically from external coupling module 20 andincorporated into a separate device (not shown) which also suppliespower to external module 20 via flexible electrical cable 475, permanentdata storage for permanently recording pressure as a function of timeand an Ethernet network interface for continuous network monitoring ofthe patients intracranial pressure.

FIGS. 15 a and 15 b describe the method used by microprocessor 440 tocompute intracranial pressure P. In FIG. 15 a, measured “Dipper”amplitude 433 traces the absorption of RF energy by the tuned referencecircuit 530 and the tuned sensor circuit 520. The dips in thefrequency-amplitude function 482 correspond to the resonant frequenciesof the implanted tuned circuits: the lower frequency dip 486 atfrequency ƒ₁ corresponds to the sensor circuit 520 at the intracranialpressure P, and the higher frequency dip 485 at frequency ƒ₀ correspondsto reference circuit 530 at the reference pressure P₀. Microprocessor440, under programmatic control, computes the frequency difference(ƒ₀−ƒ₁) between the two minima of the frequency-amplitude function 482.

As shown in FIG. 15 b, the intracranial pressure P applied to sensorcrystal 521 is a decreasing linear function of the measured frequencydifference (ƒ₀−ƒ₁) and is calculated by microprocessor 440 according tothe formula P=P₀−m(ƒ₀−ƒ₁) where the slope m is determined by thespecific geometry and physical characteristics of the sensor crystal 521and P₀ is the pressured applied to the sealed reference crystal 531. Theslope m is measured post-assembly and prior to subcutaneous insertion ina two-point calibration process. The slope m and fixed pressure P₀,resonant frequency f₀ and resonant frequency f₁ at ambient air pressureare calibration parameters recorded in the RF-ID tagging device 542.

In the alternate embodiment where magnetic switch 537 connects inductivecoil 536 to either sensor crystal 521 or to reference crystal 531,magnetic switch 537 is first reset to connect only the reference crystal531 to inductive coil 536. “Dipper” circuit 430 is scanned to read theresonant frequency ƒ₀ and then magnetic switch 537 is set to connectsensor crystal 521 to inductive coil 536. “Dipper” circuit 430 is thenscanned to read sensor frequency ƒ₁. The difference (ƒ₀−ƒ₁) iscalculated and the pressure P computed from P=P₀−m(ƒ₀−ƒ₁).

In the preferred embodiment of the present invention, a suitable choicefor microprocessor 440 and ADC 435 is the Microchip part numberPICHJ128GP306 which is a microcontroller that contains an onboardanalog-to-digital converter (ADC), two on-board Universal Asynchronoustransceivers (UARTs) for communications, an onboard pulse-widthmodulation (PWM) output for control, and several onboard timer counters.The onboard PWM of the PIC microcontroller may be used in conjunctionwith an external RC integrator to form digital-to-analog converter (DAC)455. Frequency counter 450 may be realized by using one of the on-boardtimer counters of the PIC microcontroller with a suitable frequencydivider. A suitable part for voltage-controlled oscillator (VCO) 460 isthe 74HCT4046 phase locked loop (PLL) from Texas Instruments or a numberof other semiconductor vendors. The output of the 74HCT4046 is typicallybuffered to achieve a 50 ohm drive capability. A suitable referencedesign for RCVR 445 can be found in the Microchip MicroID® 13.57 MHzSystem Design Guide located on the website www.microchip.com. Readoutdevice 480 can be any number of LCD panels from a number of suppliers,an example being part number DMC20434N-EP made by Optrex Corporation.

The preferred embodiment of “Dipper” circuit 430 is shown in the circuitdiagram of FIG. 16. With reference then to FIGS. 14 and 16, VCO 460 RFoutput signal 465 is coupled into the circuit via coupling capacitor 610and coupling capacitor 615. Resistor 620 acts to match the outputimpedance of VCO 460; both resistor 620 and VCO 460 output impedance actas a load to a resonant LC circuit comprised of capacitor 625 andinductor 425. Inductor 425 is a pluggable coil connected throughelectrical mount points 630 a and 630 b and positioned physically tomaximize the electromagnetic field coupling to sensor circuit 520 andreference circuit 530. Inductor 425, capacitor 625 and resistor 620 aretogether tied to a common ground.

Diode 635 functions to produce a DC voltage in proportion to the RFsignal current across inductor 425. Said DC voltage is transferred tothe right half of “Dipper” circuit 430 through RF choke 640 which, incombination with bypass capacitor 645, effectively isolates highfrequency RF signals from the DC amplifier part of the circuit nearoperational amplifier 650. Note that capacitor 615 functions to block DCvoltage present at diode 635 from the resonant LC circuit and capacitor610 functions to block said DC voltage from VCO 460. An invertingamplifier, comprised of input resistor 655 of resistance R_(i), feedbackresistor 665 of resistance R_(f) and operational amplifier 650,amplifies the DC voltage generated by diode 635 to form the “Dipper”amplitude 433 which is a voltage sensed by ADC 435. The gain of saidinverting amplifier is approximately the negative ratio of the feedbackresistance 665 to the input resistance 655 and has a valueG=−R_(f)/R_(i)˜100 to match the input dynamic range of ADC 435. Sincethe diode 635 DC voltage is nominally −40 mV, “Dipper” amplitude 433 isnominally 4 volts positive.

As the varying frequency of VCO 460 approaches one of the two resonancesof sensor circuit 520 or reference circuit 530, the RF energy in theresonant LC circuit (of inductor 425 and capacitor 625) decreases andthe DC voltage at diode 635 will drop correspondingly as will itsamplified version “Dipper” amplitude 433.

In the preferred embodiment, inductor 425 and capacitor 635 are chosento have values of 2 μH and 120 pF, typically. This provides for areasonably broad resonance frequency response with a peak at 10 MHz andQ of 3 so that the resonance frequencies of the implanted devices may bereadily scanned. Resistor 620 is nominally 50 ohm coinciding with theoutput impedance of VCO 460. Capacitor 615 is approximately 1000 pF andcapacitor 610 is approximately 2000 pF. A 1IN5711 Shottky barrier diodeis a suitable choice for diode 635. RF choke 640 is nominally 2.2 mH andbypass electrolytic capacitor 645 is 0.1 μF. Resistor 655 is chosen tobe 6.7 k-ohm while resistor 665 is 670 k-ohm for an inverting gain of100. Operational amplifier 650 may be an inexpensive general purposeop-amp such as part number LM741CN from National Semiconductor.

In an alternate embodiment of the present invention the gain of thefinal DC amplifier section that produces “Dipper” amplitude 433 may beuser programmable to easily accommodate varying coupling efficienciesbetween the implant 14 and the external module 20.

In the preferred embodiment, inductor 425 and capacitor 625 have typicalvalues of approximately 2 uH and 250 pF. These selections provide for areasonably broad resonance frequency response with a peak at 8 Mhz and Qof 1.6 so that the resonance frequencies of the implanted sensor andreference circuits may be readily scanned.

Referring to FIG. 17, a preferred embodiment is shown. Epoxy endcap 540comprises a housing for inductive coil 525 and inductive coil 535, andfor interconnects 523, 524, 533 and 534 which function to interconnectthe PZT substrate 510 electrically to said inductive coils. Epoxy endcap540 also houses RF-ID tagging device 542 for identification purposes anda third inductive coil (not shown) which is connected to RF-ID device542 for RF-ID powering and interrogation.

In the preferred embodiment, the epoxy endcap is a cylindrical containerwhich can possess annular threads for use to secure the epoxy endcap inthe skull.

External casing 550 and internal casing 1730 of implant 14 areconstructed of a biocompatible metal such as titanium or alloy thereofor alternatively constructed of a biocompatible plastic. External casing550 contains fluid ports 551 and 552 for allowing fluid to flow into andout of the ambient pressure cavity 560 so that said cavity is inpressure equilibrium with the intracranial fluid. Shoulder 554 ismachined on the inside of external casing 550 about midway along itslength. Internal casing 1730 is mounted inside external casing 550against shoulder 554 and is held firmly in place by the epoxy endcap540. Internal casing 1730 is gold sputtered which allows for the use ofsolder for electrical and mechanical attachment. PZT crystal substrate510 is attached to internal casing 1730 by hermetically sealed fillet565 and by solder. Alternatively, the internal casing 1730 may becomposed of a biocompatible plastic which is hermetically sealed viahermetically sealed fillet 565. The metallization 514 may be extended asa small tab or short distance into reference cavity 570 to allowsoldering of connector 533 directly to metallization 514. The externalcasing is cylindrical in form having a hemispherical dome opposite theepoxy endcap. The internal casing 1730 is generally cylindrical having acrystal support disc 1731 adjacent and supporting the center of the PZTcrystal substrate 510. Internal casing 1730 also has a base support disc532 which, when assembled, is secured within epoxy endcap 540.Hermetically sealed fillet 565 serves to rigidly connect crystal supportdisc 1731 and PZT crystal substrate 510.

The tubular shaped PZT crystal 510 is metalized to form two functionallyindependent resonating devices, namely the reference crystal 531 and thesensor crystal 521. The sensor crystal 521 is formed in contact withambient pressure cavity 560 while the reference crystal 531 is formed incontact with reference cavity 570. The interior surface of PZT crystal510 is metalized along its entire length with a common metallizationlayer 518. Wire leads are soldered directly to common metallizationlayer 518 and are connected to inductive coil 525 and inductive coil 535via interconnects 523 and 524, respectively. Endcap 540 may behermetically sealed to the PZT crystal substrate 510 so that the ambientpressure applies only to the external surface of the PZT crystal. Theinterior of the PZT crystal is at the same pressure as the referencepressure cavity 570 in the preferred embodiment of the presentinvention.

The exterior surface of PZT crystal 510 is metalized in two segments: afirst segment, transducer metallization layer 514, which extends fromendcap 512 to the vicinity of shoulder 554 and a second segmentreference metallization layer 516, which extends from the rightmost end(as shown in FIG. 17) of PZT crystal 510 near epoxy endcap 540 to thevicinity of internal casing 1730, reference metallization layer 516being etched so that it does not come into electrical contact withinternal casing 1730 or transducer metallization layer 514.

Transducer metallization layer 514 is in contact with internal casing1730 so that an electrically conductive path exists from transducermetallization layer 514 along the internal casing 1730 into the vicinityof the epoxy endcap 540. Interconnect 533 connects tranducer inductivecoil 525 to internal casing 1730 and thus to the transducermetallization layer 514. Interconnect 534 is soldered to referencemetallization 516 and connected to inductive coil 535. In an alternateembodiment where the internal casing 1730 and external casing 550 areboth made of biocompatible plastic material and hermetically sealed viahermetically sealed fillet 565, the metallization 514 may be extended asa small tab a short distance into reference cavity 570 to allowsoldering of interconnect 533 directly to metallization 514.

Gold is utilized for metallization to provide biocompatibility andminimize deposition of bioproteins on the sensor. Wire leads may besoldered directly to the metalized layer. Hermetic seals may be composedof medical grade epoxy, silicone or other suitable material.

In the preferred embodiment of the present invention, external couplingmodule 20 is located on a printed circuit board (PCB) in a moldedplastic housing which also houses inductive coil 425 and readout device480, an LCD panel attached to the given PCB circuit board. A holder forbatteries and power-on button are included with said molded plastichousing. When powered, microprocessor 440 boots up and thenautomatically operates to scan the “Dipper” amplitude 433 and locatenearby resonant circuits. Molded plastic housing has tabs for placingthe unit onto the patient's head and securing with straps or with tape.

In practice, microprocessor 440 scans the dipper frequencies by firstsending the appropriate signals to excite the transducer section andreference section via inductive coil 425. The frequency absorption ofthe transducer section and the reference section of the PZT crystalsubstrate are then measured and compared to determine a frequencydifference. The frequency difference is relayed to the microprocessorwhich then relates the difference in frequency to the reference pressureto determine the intracranial fluid pressure according to the equationP=P0−m)ƒ₀−ƒ₁) as previously described.

Additionally, the microprocessor is programmed to store a set ofcalibrated data in the memory of the RF-ID tagging device 542 includingthe initial reference pressure, the initial intracranial pressure andthe initial calibration slope. The microprocessor is also programmed tostore a set of patient data in the memory of the RF-ID tag and devicesuch as name, social security number, relevant medical conditions andother relevant patient data. Empirical equations or tables relatingprotein deposits to crystal resonant frequency may also be stored in theRF-ID tag.

In an alternate embodiment, the microprocessor can be configured tocompensate for protein deposits as follows. In practice, the ICP sensorcan be implanted in a patient for many years. Since the transducercrystal is physically exposed to intracranial fluid during this entireperiod of time, a protein buildup is expected on its surface. Theprotein buildup serves to slow the vibration of the transducer crystaland increase the power required to accomplish oscillation. Referring toFIG. 19, the local minimum of the sensor q₀ can be seen to increase fromq₀ to q₁ after an elapsed period of time, t_(elapsed) due to proteindeposits. Further, FIG. 19 shows that the frequency of the sensor driftsfrom ƒ₁ to ƒ_(t) due to the same phenomenon.

Referring to FIG. 20, a graph showing sensor frequency versus timeaccording to protein deposits is shown. At initial time t₀, a localminimum sensor frequency ƒ₁ is shown. After an elapsed time is equal tot_(elapsed) a frequency shift to ƒ_(t) is shown. An empirical equationcan be derived for any time t to report a frequency shift ƒ_(r)ƒ_(t).The microprocessor is programmed to report a frequency ƒ_(r) accordingto the equation ƒ_(reported)=ƒ_(measured)+(ƒ₁−ƒ_(t); where ƒ_(measured)is the frequency minimum reported by the dipper circuit, ƒ₁ is theinitial local minimum of the sensor and ƒ_(t) is the frequency derivedfrom the elapsed time sensor frequency curve stored in themicroprocessor. A lookup table for an empirical equation can be employedby the microprocessor to arrive at ƒ_(t) given t_(elapsed).

In order to derive to t_(elapsed), the microprocessor stores the initialdate and time of the implant of the ICP sensor in the patient in RF-IDtagging device 542 as initial time t₀. When the system is initiated andreadings are taken after implant, the microprocessor subtracts t₀ fromthe current date and time to arrive at t_(elapsed).

In a second embodiment of the housing for the present invention, thedisplay device and power supply is contained in a separate instrumenthousing connected to external coupling module 20, itself housed on a PCBcircuit board in a molded plastic housing along with inductive coil 425and readout device 480. Said instrument housing is connected via a cablewith wires sufficient for power and for a serial interface, the latterbeing connected to an onboard UART built into microprocessor 440 forserial communications. A permanent storage device, such as hard drive,CD R/W or DVD R/W is included in the instrument housing which also hasan Ethernet network interface for network-based monitoring ofintracranial pressure.

While the present invention has been described in terms of specificembodiments thereof, it will be understood in view of the presentdisclosure, that numerous variations upon the invention are now enabledto those skilled in the art, which variations yet reside within thescope of the present teaching. Accordingly, the invention is to bebroadly construed, and limited only by the scope and spirit of theclaims now appended hereto.

1. An implantable biometric sensor comprising: a subcutaneous case; anaccess portal in the case to an internal fluid; a sensor within the casegenerating a signal related to a property of the internal fluid; aconversion circuit within the case transforming the signal into atransmitted near infrared optical form; and an inductive power couplingproviding power to the sensor and the conversion circuit.
 2. Theimplantable biometric sensor of claim 1 wherein the sensor is anoscillated crystal comparator.
 3. The implantable biometric sensor ofclaim 1 wherein the oscillated crystal comparator comprises: an isolatedreference crystal connected to a reference oscillator; a transducercrystal connected to a transducer oscillator; and a heterodyne amplifierconnected to the transducer oscillator and the reference oscillator. 4.The implantable biometric sensor of claim 3 wherein the isolatedreference crystal is contained within a pressure sealed container.
 5. Animplantable biometric sensor system comprising: a rigid subcutaneouscase; an access portal to an internal fluid in the rigid subcutaneouscase; a dual frequency crystal oscillator, affixed to the rigidsubcutaneous case and in ducted communication with the internal fluid,for sensing a pressure difference in the internal fluid; and at leastone driver coil in electrical communication with the dual frequencycrystal oscillator.
 6. The implantable biometric sensor system of claim5 wherein the dual frequency crystal oscillator includes a pair ofseparated piezoelectric crystals.
 7. The implantable biometric sensorsystem of claim 5 wherein the dual frequency crystal oscillator includesa pair of physically integrated piezoelectric crystals.
 8. Theimplantable biometric sensor system of claim 5 wherein the dualfrequency crystal oscillator includes a single cylindrical crystalsubstrate electrically separated into a pair of electrically isolatedcrystals.
 9. The implantable biometric sensor system of claim 5 whereinthe dual frequency crystal oscillator oscillates in a hoop mode.
 10. Theimplantable biometric sensor system of claim 5 wherein the driver coilis comprised of a pair of matched inductors electrically connected tothe dual frequency crystal oscillator.
 11. The implantable biometricsensor system of claim 5 wherein the dual frequency crystal oscillatorcomprises a set of matched cylindrical crystal oscillators and thedriver coil comprises a set of matched inductors connected to the set ofmatched cylindrical crystal oscillators.
 12. The implantable biometricsensor system of claim 5 wherein the rigid subcutaneous case includes: acylindrical rigid probe; and a rigid support structure adjacent thecylindrical rigid probe suspending the dual frequency crystal oscillatorin ducted communication with the internal fluid and in ductedcommunication with a reference pressure cavity.
 13. The implantablebiometric sensor system of claim 5 further comprising: a support baseconnecting the rigid probe and the rigid support structure and whereinthe support base houses the driver coil.
 14. The implantable biometricsensor system of claim 5 wherein the dual frequency crystal oscillatorcomprises: a cylindrical crystal, having an exterior surface and aninterior surface, supported centrally by the subcutaneous case; a firstmetalized cylinder adjacent the exterior surface; a second metalizedcylinder adjacent the exterior surface; a third metalized cylinderadjacent the interior surface; a first inductive coil supported by thesubcutaneous case electrically connected to the first metalized cylinderand the third metalized cylinder; and a second inductive coil supportedby the subcutaneous case electrically connected to the second metalizedcylinder and the third metalized cylinder.
 15. The implantable biometricsensor system of claim 5 wherein the dual frequency crystal oscillatorcomprises: a first resonant inductor crystal circuit in communicationwith the internal fluid; a second resonant inductor crystal circuit incommunication with a reference pressure cavity; an external couplingmodule for driving the first and second resonant inductor crystalcircuits and recovering a radio frequency energy null corresponding to aresonant frequency for each of the first and second resonant inductorcrystal circuits.
 16. The implantable biometric sensor system of claim15 wherein the external coupling module further comprises: a driver coilconnected to a dipper circuit; a voltage controlled oscillator connectedto the dipper circuit; a digital to analog converter connected to thevoltage controlled oscillator and a microprocessor; a frequency counterconnected to the voltage controlled oscillator and the microprocessor;an analog to digital converter connected to the dipper circuit and themicroprocessor; wherein the microprocessor is programmed to carry outthe steps of: supplying a first digital signal to the digital to analogconverter representing a desired frequency sweep; reading the frequencysweep from the frequency counter; reading a second digital signal fromthe analog to digital converter representing an output of the dippercircuit; and translating the second digital signal into a representationof a biometric parameter.
 17. The implantable biometric sensor system ofclaim 16 wherein the microprocessor is programmed to carry out theadditional step of adjusting the representation of the biometricparameter for a protein deposit.
 18. The implantable biometric sensorsystem of claim 5 wherein the dual frequency crystal oscillatorcomprises: a first tubular crystal connected to the at least one drivercoil through an externally controllable switch; a second tubular crystalconnected to the at least one driver coil through the externallycontrollable switch; and wherein the externally controllable switch isconfigured to alternatively electrically connect the first tubularcrystal to the at least one driver coil and the second tubular crystalto the at least one driver coil.
 19. A method of determiningintracranial fluid pressure providing the steps of: providing acylindrical crystal oscillator probe having a transducer section and areference section; providing at least one driver circuit within thecylindrical crystal oscillator probe connected to the transducer sectionand the reference section; exposing the reference section to a referencepressure; exposing the transducer section to the intracranial fluidpressure; exciting the transducer section to provide a first radiofrequency energy absorption; exciting the reference section to produce asecond radio frequency energy absorption; measuring the first radiofrequency energy absorption; measuring the second radio frequency energyabsorption; comparing the measurement of the first radio frequencyenergy absorption and the measurement of the second radio frequencyenergy absorption to determine a difference; relaying the difference tothe reference pressure to determine the intracranial fluid pressure; andreporting the intracranial fluid pressure.
 20. The method of claim 19wherein the steps of comparing and relating include application of theformulaP=P ₀ −m(ƒ₀−ƒ₁) where: P=intracranial pressure; P₀=reference pressure;m=calibration slope; f₀=resonant frequency of reference oscillator; andf₁=resonant frequency of transducer oscillator.
 21. The method of claim20 wherein the steps of exciting the transducer section and exciting thereference section include the steps of: sweeping an oscillatingelectromagnetic field over at least one coil connected to the referencesection and the transducer section to produce at least one frequencyabsorption null.
 22. The method of claim 19 including the further stepsof: providing a memory in the crystal oscillator probe; and storing aset of calibration data in the memory.
 23. The method of claim 19including the further steps of: providing a memory in the crystaloscillator probe; and storing a set of patient data in the memory. 24.The method of claim 19 comprising the additional step of adjusting themeasurement of the first frequency absorption to account for a proteindeposit from the intracranial fluid.
 25. The method of claim 24 whereinthe step of adjusting includes the step of applying the followingequation:ƒ_(r)=ƒ_(measured)+(ƒ₁−ƒ_(t)) where: ƒ_(measured)=the first frequencyabsorption; ƒ₁=an initial first frequency absorption; and ƒ_(t)=thefrequency derived from an elapsed time transducer section frequencycurve stored in a memory.
 26. The method of claim 25 including thefurther step of storing the initial first frequency absorption and atime of inception in the memory.